<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
  var a = '[a, b, c, d]';
  var b = [1, 2, 3, 4];
  document.write("a is ");
  document.write(a instanceof Array);
  document.write(" array"+'<br>');
  document.write("b is ");
  document.write(Array.isArray(b));
  document.write(" array");
</script>
</body>
</html>